Process for making graphite articles

ABSTRACT

A process for preparing graphite articles is presented. In particular, the process includes employing a particulate fraction comprising at least about 35 weight percent coke, coal or combinations thereof having a diameter such that a major fraction of it passes through a 0.25 mm to 25 mm mesh screen. The particulate fraction is mixed with a liquid or solid pitch binder, to form a stock blend; the stock blend is extruded to form a green stock; the green stock is baked to form a carbonized stock; and the carbonized stock is graphitized. The stock blend further includes carbon fibers added after mixing of the particulate fraction and pitch has begun.

RELATED APPLICATION

This application is a continuation-in-part of copending and commonlyassigned U.S. patent application Ser. No. 10/649,359, filed in the nameof Kortovich, Shao, Huang, Lewis and Lewis on Aug. 27, 2003, entitled“Process of Making Graphite Articles,” the disclosure of which is herebyincorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to graphite articles, and a process forpreparing the inventive graphite articles. More particularly, theinvention concerns articles such as graphite electrodes or cathodesformed by processing a blend of (i) a particulate fraction comprising atleast about 35 weight percent calcined coke and (ii) pitch, where theblend further includes small particle size filler, carbon fibers, orcombinations thereof.

2. Background Art

Graphite electrodes are used in the steel industry to melt the metalsand other ingredients used to form steel in electrothermal furnaces. Theheat needed to melt metals is generated by passing current through aplurality of electrodes, usually three, and forming an arc between theelectrodes and the metal. Electrical currents in excess of 100,000amperes are often used. The resulting high temperature melts the metalsand other ingredients. Generally, the electrodes used in steel furnaceseach consist of electrode columns, that is, a series of individualelectrodes joined to form a single column. In this way, as electrodesare depleted during the thermal process, replacement electrodes can bejoined to the column to maintain the length of the column extending intothe furnace.

Generally, electrodes are joined into columns via a pin (sometimesreferred to as a nipple) that functions to join the ends of adjoiningelectrodes. Typically, the pin takes the form of opposed male threadedsections, with at least one end of the electrodes comprising femalethreaded sections capable of mating with the male threaded section ofthe pin. Thus, when each of the opposing male threaded sections of a pinare threaded into female threaded sections in the ends of twoelectrodes, those electrodes become joined into an electrode column.Commonly, the joined ends of the adjoining electrodes, and the pin therebetween, are referred to in the art as a joint.

Given the extreme thermal stress that the electrode and the joint (andindeed the electrode column as a whole) undergoes, mechanical/thermalfactors such as strength, thermal expansion, and crack resistance mustbe carefully balanced to avoid damage or destruction of the electrodecolumn or individual electrodes. For instance, longitudinal (i.e., alongthe length of the electrode/electrode column) thermal expansion of theelectrodes, especially at a rate different than that of the pin, canforce the joint apart, reducing effectiveness of the electrode column inconducting the electrical current. A certain amount of transverse (i.e.,across the diameter of the electrode/electrode column) thermal expansionof the electrode in excess of that of the pin may be desirable to form afirm connection between pin and electrode; however, if the transversethermal expansion of the electrode greatly exceeds that of the pin,damage to the electrode or separation of the joint may result. Again,this can result in reduced effectiveness of the electrode column, oreven destruction of the column if the damage is so severe that theelectrode column fails at the joint section. Thus, control of thethermal expansion of an electrode, in both the longitudinal andtransverse directions, is of paramount importance.

As a consequence, if the pin can be eliminated from theelectrode/electrode column system, the need to balance the thermalexpansion of the different system components (i.e., pin and electrode)is reduced. Prior attempts to eliminate the pin have been attempted,where a threaded electrode end or other electrode mating means have beenemployed. Industry acceptance has lagged, however, since it is felt thatthe strength of the graphite is not sufficient to maintain the integrityof the electrode column without a pin. Regardless of whether the pin iseliminated or not, increased graphite electrode strength and toughness(which can be defined as resistance to cracking) and reduction ofbrittleness (which can be defined as the rate of propagation of cracks)is desired in order to extend electrode life.

Similarly, in the case of graphite cathodes (utilized in the aluminumsmelting industry) and other synthetic graphite artifacts, increasedstrength and toughness will result in longer life and improvedusability.

There have been references to the use of mesophase pitch-based carbonfibers to improve specific properties of bulk graphite products such aselectrodes. For instance, Singer, in U.S. Pat. No. 4,005,183, describesthe production of mesophase pitch-based fibers and states that, becauseof their low electrical resistivity, these fibers can be employed asfiller material in the production of graphite electrodes. In BritishPatent 1,526,809 to Lewis and Singer, 50% to 80% by weight of carbonfibers are added to 20% to 50% by weight of pitch binder and thenextruded to form a carbon artifact that can be graphitized. Theresulting article exhibits relatively low longitudinal thermalexpansion.

In U.S. Pat. No. 4,998,709, Griffin et al. attempt to address theproblems caused by excessive longitudinal thermal expansion of electrodepins by preparing a graphite nipple (i.e., pin) with mesophasepitch-based carbon fibers included in the extrusion blend. The carbonfibers used by Griffin et al. have a Young's modulus of greater than55×10⁶ pound-forces per square inch (psi), and are present in the blendat about 8 to 20 weight percent. The blend is extruded, baked, and thengraphitized for from about 5 to 14 days to produce the nipple. Althoughnipples produced by the Griffin et al. process show a decrease in thecoefficient of thermal expansion (CTE) in the longitudinal direction,they also show an undesirable increase in CTE in the transversedirection, an increase in electrical resistivity and a decrease in themodulus of rupture. In addition, the graphitizing time is extremely longcompared with times that would be advantageous for commercialproduction.

In an improved process for preparing connecting pins containing fibers,Shao et al. teach the inclusion of carbon fibers derived from mesophasepitch in the calcined coke/pitch blend, in U.S. Pat. No. 6,280,663. Theresulting pins exhibit reduced longitudinal CTE without requiringcommercially disadvantageous graphitizing time. However, even suchimproved pins as produced by the Shao et al. process do not eliminatethe need for electrodes with improved strength; additionally, if pinscould be eliminated altogether, the savings and efficiency gains wouldbe extremely beneficial.

What is desired, therefore, is a graphite article having reduced CTE inthe longitudinal direction as compared with art-conventional graphitearticles, without sacrificing transverse CTE or resistivity and modulusof rupture. Moreover, graphite articles having increased strength andtoughness, especially increased strength and toughness sufficient topermit mating of electrodes without the use of a pin, are also desired.It is also highly desirable to achieve these property benefits withoutusing high quantities of expensive materials.

SUMMARY OF THE INVENTION

It is an aspect of the present invention to provide a process forpreparing graphite articles.

It is another aspect of the present invention to provide a process forpreparing graphite articles, such as graphite electrodes or graphitecathodes, having reduced longitudinal coefficient of thermal expansionand improved resistance to cracking and fracture as compared toart-conventional graphite articles.

It is yet another aspect of the present invention to provide a processfor preparing graphite articles, the articles having reducedlongitudinal coefficient of thermal expansion as compared toart-conventional articles, without substantial sacrifice of transverseCTE or resistivity while also increasing the modulus of rupture.

Still another aspect of the present invention is a graphite article,such as a graphite electrode or graphite cathode, having reducedlongitudinal coefficient of thermal expansion and improved resistance tocracking and fracture as compared to art-conventional graphite articlesespecially without substantial sacrifice of transverse CTE orresistivity while also increasing the modulus of rupture.

These aspects and others that will become apparent to the artisan uponreview of the following description can be accomplished by providing aprocess for preparing graphite articles, the process including employinga particulate fraction comprising at least about 35 weight percent, andmore preferably at least about 50 weight percent, most preferably atleast about 70 weight percent, coke, coal or combinations thereof havinga diameter such that a major fraction of it passes through a 0.25 mm to25 mm mesh screen. Calcined coke is most commonly employed in theparticulate fraction. The particulate fraction is mixed with a liquid orsolid pitch binder, to form a stock blend; the stock blend is extrudedto form a green stock; the green stock is baked to form a carbonizedstock; and the carbonized stock is graphitized by heating it to atemperature of at least about 2500° C. and maintaining it at thattemperature for a suitable time. The stock blend further comprisescarbon fibers added after mixing of the particulate fraction and pitchhas begun and, optionally, small particle size filler (advantageouslyadded as part of the particulate fraction).

In the inventive process, the carbon fibers are preferably present at alevel of about 0.5 to about 6 parts by weight of carbon fibers per 100parts by weight of calcined coke, or at about 0.4% to about 5.5% byweight of the total mix components (excluding binder). The preferredfibers have an average diameter of about 6 to about 15 microns, and alength of preferably about 4 mm to about 25 mm, and most preferably lessthan about 32 mm. The carbon fibers used in the inventive process shouldpreferably have a tensile strength of at least about 150,000 psi. Mostadvantageously, the carbon fibers are added to the stock blend asbundles, each bundle containing from about 2000 to about 20,000 fibers.

As noted above, the fibers are added after mixing of the particulatefraction and pitch has already begun. Adding the fibers after the mixinghas begun will help preserve fiber length (which can be reduced duringthe mixing process) and thereby the beneficial effects of the inclusionof fibers, which are believed to be related to fiber length. In order toachieve a beneficial result, the fibers should preferably be added afterat least about 15% of the mix cycle has been completed, more preferablyafter at least about 30% of the mix cycle has been completed. Indeed, ina more preferred embodiment, the fibers are added after at least abouthalf the mix cycle has been completed, most advantageously after atleast about three-quarters of the mix cycle has been completed. Forinstance, if the mixing of the particulate fraction and pitch takes twohours (i.e., a mix cycle is two hours), the fibers should be added afterat least about 18 minutes of mixing (or 15% of the two hour mix cycle).

As noted above, the particulate fraction can include small particle sizefiller (small is used herein as compared to the particle size of thecalcined coke, which generally has a diameter such that a major fractionof it passes through a 25 mm mesh screen but not a 0.25 mm mesh screen,and as compared to the fillers conventionally employed). Morespecifically, the small particle size filler comprises at least about75% coke powder, by which is meant coke having a diameter such that atleast about 70% and more advantageously up to about 90%, will passthrough a 200 Tyler mesh screen, equivalent to 74 microns.

The small particle size filler can further comprise at least about 0.5%and up to about 25% of other additives like a puffing inhibitor such asiron oxide. Again, the additive should also be employed at a particlesize smaller than that conventionally used. For instance, when ironoxide is included, the average diameter of the iron oxide particlesshould be such that they are smaller than about 10 microns. Anotheradditional additive which can be employed is petroleum coke powder,having an average diameter such that they are smaller than about 10microns, added to fill porosity of the article and thus enable bettercontrol of the amount of pitch binder used. The small particle sizefiller should comprise at least about 30%, and as high as about 50% oreven 65% of the particulate fraction.

The baking of the green stock preferably takes place at a temperature ofup to about 700 to about 1000° C. in a non-oxidizing or reducingenvironment, and graphitization is preferably at a temperature of fromabout 2500 to about 3400° C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As noted above, graphite articles (graphite articles is used herein toinclude at least graphite electrodes, pins for graphite electrodes, andcathodes) can be fabricated by first combining a particulate fractioncomprising calcined coke (when the graphite article to be produced is agraphite electrode), pitch and mesophase pitch or PAN-based carbonfibers into an stock blend. More specifically, crushed, sized and milledcalcined petroleum coke is mixed with a coal-tar pitch binder to formthe blend. The particle size of the calcined coke is selected accordingto the end use of the article, and is within the skill in the art.Generally, in graphite electrodes for use in processing steel, particlesup to about 25 millimeters (mm) in average diameter are employed in theblend. The particulate fraction preferable includes a small particlesize filler comprising coke powder. Other additives that may beincorporated into the small particle size filler include iron oxides toinhibit puffing (caused by release of sulfur from its bond with carboninside the coke particles), coke powder and oils or other lubricants tofacilitate extrusion of the blend.

The particulate fraction can comprise other than calcined coke as the“large” particle fraction. For instance, when the article is a graphite(by which term is included semi-graphitic) cathode, the coke employedcan be calcined coke, or petroleum coke, coal derived coke, andcombinations of these cokes. The manufacture of the cathode may alsoinclude calcined anthracite coal instead of the coke or along with thecoke.

Also included in the blend are mesophase pitch-based carbon fibers orfibers derived from PAN (polyacrylonitrile), added after mixing of thestock has already begun. Graphitized fibers can also be employed. Thefibers used should advantageously have a Young's modulus (aftercarbonization) of about 15×10⁶ psi to about 40×10⁶ psi. They preferablyhave an average diameter of about 6 to about 15 microns, a tensilestrength of about 200×10³ psi to about 400×10³ psi, and are preferablyabout 4 mm to about 32 mm in length on average. Suitable lengths offiber include an average length of about 6 mm or less, about 12 mm orless, about 18 mm or less, or about 25 mm or less. It is also preferredthat the carbon fibers are not longer than the biggest coke particle.Most advantageously, the fibers are added to the blend as bundlescontaining between about 2000 and about 20,000 fibers per bundle,compacted with the use of a sizing.

As noted, the carbon fibers to be included in the blend are based onmesophase pitch or PAN. Mesophase pitch fibers are produced from pitchthat has been at least partially transformed to a liquid crystal, orso-called mesophase, state. Mesophase pitch can be prepared fromfeedstocks such as heavy aromatic petroleum streams, ethylene crackertars, coal derivatives, petroleum thermal tars, fluid cracker residuesand pressure treated aromatic distillates having a boiling range from340° C. to about 525° C. The production of mesophase pitch is describedin, for example, U.S. Pat. No. 4,017,327 to Lewis et al., the disclosureof which is incorporated herein by reference. Typically, mesophase pitchis formed by heating the feedstock in a chemically inert atmosphere(such as nitrogen, argon, helium or the like) to a temperature of about350° C. to 500° C. A chemically inert gas can be bubbled through thefeedstock during heating to facilitate the formation of mesophase pitch.For preparation of carbon fibers, the mesophase pitch should have asoftening point, that is, the point at which the mesophase pitch beginsto deform, of less than about 400° C. and usually less than about 350°C. If the pitch has a higher softening point, formation of carbon fibershaving the desired physical properties is difficult.

Once the mesophase pitch is prepared, it is spun into filaments of thedesired diameter, by known processes such as by melt spinning,centrifugal spinning, blow spinning or other processes which will befamiliar to the skilled artisan. Spinning produces carbon fiberssuitable for use in preparing the electrode of the present invention.The filaments are then thermoset at a temperature no higher than thesoftening point of the pitch (but usually above 250° C.) for about 5 to60 minutes, then further treated at extremely high temperatures, on theorder of up to about 1000° C. and higher, and in some cases as high asabout 3000° C., more typically about 1500° C. to 1700° C., to carbonizethe fibers. The carbonization process takes place in an inertatmosphere, such as argon gas, for at least about 0.5 minutes. Mostcommonly, carbonization uses residence times of between about 1 and 25minutes. The fibers are then cut to length and formed into bundles. Suchfibers, bundled as described, are commercially available from, forinstance, Cytec Industries Inc. of West Paterson, N.J. and MitsubishiChemical Functional Products Inc. of Tokyo, Japan.

One method of making the PAN fibers comprises spinning the fibers from asolution of polyacrylonitrile. The fibers are then stabilized in thesame manner as are the mesophase pitch-based fibers. The production ofPAN fibers is described, for instance, by Dan D. Edie and John J. McHughin “High Performance Carbon Fibers” at pages 119-138 of Carbon Materialsfor Advanced Technologies, 1st Ed., Elsevier Science Ltd. 1999, thedisclosure of which is incorporated herein by reference.

The carbon fibers are preferably included in the stock blend at a levelof about 0.5 to about 6 parts by weight of carbon fibers per 100 partsby weight of calcined coke. Most preferably, the fibers are present at alevel of about 1.25 to about 6 parts by weight fibers per 100 parts byweight of coke. In terms of the blend as a whole (excluding binder), thecarbon fibers are incorporated at a level of about 1% to about 5.5% byweight, more preferably about 1.5% to up to about 5.5%, even morepreferably, about 5.0% or less.

After the blend of particulate fraction, pitch binder, carbon fibers,etc. is prepared, the body is formed (or shaped) by extrusion though adie or molded in conventional forming molds to form what is referred toas a green stock. The forming, whether through extrusion or molding, isconducted at a temperature close to the softening point of the pitch,usually about 100° C. or higher. Although the die or mold can form thearticle in substantially final form and size, machining of the finishedarticle is usually needed, at the very least to provide structure suchas threads. The size of the green stock can vary; for electrodes thediameter can vary between about 220 mm and 700 mm. With respect tocathodes, a rectangular cross-section may be employed.

After extrusion, the green stock is heat treated by baking at atemperature of between about 700° C. and about 1100° C., more preferablybetween about 800° C. and about 1000° C., to carbonize the pitch binderto solid pitch coke, to give the article permanency of form, highmechanical strength, good thermal conductivity, and comparatively lowelectrical resistance, and thus form a carbonized stock. The green stockis baked in the relative absence of air to avoid oxidation. Bakingshould be carried out at a rate of about 1° C. to about 5° C. rise perhour to the final temperature. After baking, the carbonized stock may beimpregnated one or more times with coal tar or petroleum pitch, or othertypes of pitches or resins known in the industry, to deposit additionalcoke in any open pores of the stock. Each impregnation is then followedby an additional baking step.

After baking, the carbonized stock, is then graphitized. Graphitizationis by heat treatment at a final temperature of between about 2500° C. toabout 3400° C. for a time sufficient to cause the carbon atoms in thecoke and pitch coke binder to transform from a poorly ordered state intothe crystalline structure of graphite. Advantageously, graphitization isperformed by maintaining the carbonized stock at a temperature of atleast about 2700° C., and more advantageously at a temperature ofbetween about 2700° C. and about 3200° C. At these high temperatures,elements other than carbon are volatilized and escape as vapors. Thetime required for maintenance at the graphitization temperature usingthe process of the present invention is no more than about 18 hours,indeed, no more than about 12 hours. Preferably, graphitization is forabout 1.5 to about 8 hours.

As noted, once graphitization is completed, the finished article can becut to size and then machined or otherwise formed into its finalconfiguration. The articles prepared in accordance with the presentinvention exhibit a substantial reduction in longitudinal CTE ascompared with articles prepared by conventionally process. The articlesan increase in flexural strength (i.e., modulus of rupture) and anincrease in Young's modulus, without a concomitant significant increasein transverse CTE or specific resistance, without the requirement ofcommercially disadvantageous graphitization times. In addition, anincreased resistance to cracking or fracture as indicated by decreasedbrittleness and increased toughness is also observed.

The following examples are presented to further illustrate and explainthe present invention and should not be viewed as limiting in anyregard. Unless otherwise indicated, all parts and percentages are byweight, and are based on the weight of the product at the particularstage in processing indicated.

EXAMPLE 1

A graphite electrode trial was conducted with additions of fibers fromMitsubishi Chemical (mesophase pitch fibers, 18 mm long choppedbundles), Cytec (mesophase pitch fibers, 6 mm and 25 mm long choppedbundles), and Zoltek (PAN based fibers, 25 mm long chopped bundles). Theconcentration of the fiber bundles in the mix (excluding binder) wasbetween about 2.5 to about 5 weight percent. The stocks were prepared ina paddle arm, cylinder mixer, cooled, and extruded to about 150 mm×about330 mm long electrodes. The electrodes were processed as describedabove. The physical properties of the electrodes with fibers arecompared to those of control electrodes (no fibers) below. TABLE IProperties of Cylinder Mixed Electrodes With Fiber Additions DensityResistance Modulus Flex Str Long CTE Trans CTE (g/cm³) (μΩm) (psi × 10⁶)(psi) (1/° C. × 10⁻⁶) (1/° C. × 10⁻⁶) Without fibers 1.692 5.52 1.411511 0.29 1.36 Mitsubishi, 18 mm, 2.5% 1.689 5.57 1.57 1700 0.18 1.38Mitsubishi, 18 mm, 5% 1.693 5.45 1.73 1907 0.07 1.45 Cytec, 6 mm, 2%1.705 5.79 1.56 1652 0.21 1.41 Cytec, 6 mm, 4% 1.710 5.52 1.78 1926 0.121.43 Cytec, 25 mm, 2.5% 1.686 5.56 1.54 1715 0.18 1.39 Zoltek, 25 mm, 2%1.710 5.60 1.53 1574 0.19 1.47

EXAMPLE 2

A second graphite electrode trial was conducted with additions of fibersfrom Mitsubishi Chemical (mesophase pitch fibers, 30 mm long choppedbundles), Zoltek (PAN based fibers, 51 mm long chopped bundles), Cytec(mesophase pitch fibers, 6 mm and 25 mm long chopped bundles), andConocoPhillips (mesophase pitch fibers, 25 mm long chopped mat).Addition levels of the fiber bundles was about 1.5 and about 3 weightpercent. The stocks were prepared in a double arm, Sigma blade mixer,cooled, and extruded to about 150 mm×about 330 mm long electrodes. Theelectrodes were processed as described above. The physical properties ofthe electrodes with fibers are compared to those of control electrodes(no fibers) below. TABLE II Properties of Sigma Mixed Electrodes WithFiber Additions Density Resistance Modulus Flex Str Long CTE Trans CTE(g/cm³) (μΩm) (psi × 10⁶) (psi) (1/° C. × 10⁻⁶) (1/° C. × 10⁻⁶) WithoutFibers 1.658 5.98 1.18 1340 0.40 1.32 Mitsubishi, 30 mm, 1.5% 1.656 5.871.40 1515 0.21 1.25 Mitsubishi, 30 mm, 3% 1.625 5.94 1.40 1624 0.08 1.15Zoltek, 51 mm, 1.5% 1.654 5.97 1.40 1686 0.26 1.29 Zoltek, 51 mm, 3%1.634 5.85 1.42 1756 0.16 1.20 Cytec, 6 mm, 1.5% 1.641 6.12 1.33 15310.23 1.18 Cytec, 6 mm, 3% 1.611 6.01 1.38 1667 0.11 1.17 Cytec, 25 mm,1.5% 1.627 6.27 1.23 1488 0.23 1.22 Cytec, 25 mm, 3% 1.624 6.00 1.411706 0.10 1.16 Conoco, 25 mm, 1.5% 1.648 6.07 1.32 1458 0.21 1.19Conoco, 25 mm, 3% 1.620 5.85 1.40 1560 0.04 1.14

EXAMPLE 3

A third graphite electrode trial was conducted with additions of thefibers from Mitsubishi Chemical only (mesophase pitch fibers, 6 mm longchopped bundles), the same fibers as used in U.S. Pat. No. 6,280,663.Addition levels were 2, 4, and 6 weight percent. The stocks were againprepared in the paddle arm, cylinder mixer, cooled, and extruded to 150mm×330 mm long electrodes. The electrodes were processed as describedabove. The physical properties of the electrodes with fibers arecompared to those of control electrodes (no fibers) below. TABLE IIIProperties of Cylinder Mixed Electrodes With Fiber Additions DensityResistance Modulus Flex Str Long CTE Trans CTE (g/cm³) (μΩm) (psi × 10⁶)(psi) (1/° C. × 10⁻⁶) (1/° C. × 10⁻⁶) Without Fibers 1.685 5.25 1.221323 0.25 1.24 Mitsubishi, 6 mm, 2% 1.692 5.07 1.44 1534 0.11 1.21Mitsubishi, 6 mm, 4% 1.685 5.12 1.52 1676 0.06 1.24 Mitsubishi, 6 mm, 6%1.684 5.13 1.59 1715 −0.01 1.15

EXAMPLE 4

A fourth graphite electrode trial was conducted with additions of thefibers from Mitsubishi Chemical (mesophase pitch fibers, 6 mm and 25 mmlong chopped bundles). Addition level was 5%. The stocks were preparedin the Sigma mixer, cooled, and extruded to 150 mm×330 mm longelectrodes. The filler size (coke powder and iron oxide) was eitherstandard or fine (coke powder 55% finder than 74 microns or 90% finerthan 74 microns, iron oxide 5 microns rather than 74 microns, extra cokefines (1-10 microns) added to the batch). Also, some of the batches wereprepared with the fibers added after 50 minutes of the 70 minuteheating/mixing cycle rather than at the beginning. The electrodes wereprocessed as described above. The physical properties of the electrodeswith fibers are compared to those of control electrodes (no fibers,either standard size or fine filler) below. TABLE IV Properties of SigmaMixed Electrodes With Fine Fillers and Fiber Additions filler fiber FlexLong. Trans. & oxide charging Dens. Resist. Str. CTE CTE Britt. Toughsizing time (g/cm3) (uhm) (psi) (/C. × 10−6) (/C. × 10−6) (kN/mm) (mm)Control, 0″, .0% std none 1.608 6.28 925 0.116 1.060 3.02 0.79 FineFiller, 0″, .0% fine none 1.582 7.13 732 0.075 0.940 1.79 1.12 Fibersfine initial 1.553 6.11 1154 −0.296 0.782 6.11 0.66 & Fine Filler,0.25″, 5% Fibers fine 50 min 1.545 6.27 1134 −0.370 0.744 4.16 0.94 &Fine Filler, 0.25″, 5% Fibers fine initial 1.588 5.60 1261 −0.292 0.8396.51 0.57 & Fine Filler, 1″, 5% Fibers fine 50 min 1.562 6.16 1146−0.472 0.635 2.30 1.57 & Fine Filler, 1″, 5%

The disclosures of all cited patents and publications referred to inthis application are incorporated herein by reference.

The above description is intended to enable the person skilled in theart to practice the invention. It is not intended to detail all of thepossible variations and modifications that will become apparent to theskilled worker upon reading the description. It is intended, however,that all such modifications and variations be included within the scopeof the invention that is defined by the following claims. The claims areintended to cover the indicated elements and steps in any arrangement orsequence that is effective to meet the objectives intended for theinvention, unless the context specifically indicates the contrary.

1. A process for preparing a graphite article, the process comprising(a) mixing (i) a particulate fraction comprising at least about 35weight percent coke, coal or mixtures thereof having a diameter suchthat a major fraction passes through a 25 mm mesh screen but not a 0.25mm mesh screen, (ii) a pitch binder and (iii) carbon fibers, to form astock blend; (b) extruding the stock blend to form a green stock; (c)baking the green stock to form a carbonized stock; and (d) graphitizingthe carbonized stock by maintaining the carbonized stock at atemperature of at least about 2500° C., wherein the fibers are added tothe stock blend after mixing has begun.
 2. The process of claim 1wherein the fibers have an average length of no more than about 32 mm.3. The process of claim 1 wherein the fibers are added to the stockblend after at least about 15% of the mix cycle is completed.
 4. Theprocess of claim 3 wherein the fibers are added to the stock blend afterat least about 50% of the mix cycle is completed.
 5. The process ofclaim 1 wherein the carbon fibers are present at a level of about 0.5 toabout 6 parts by weight of carbon fibers per 100 parts by weight ofcalcined coke.
 6. The process of claim 5 wherein the carbon fibers havea tensile strength of at least about 150,000 psi.
 7. The process ofclaim 5 wherein the carbon fibers have a Young's modulus of about 15×10⁶psi.
 8. The process of claim 5 wherein the carbon fibers have an averagediameter of about 6 microns to about 15 microns.
 9. The process of claim1 wherein the particulate fraction comprises materials selected from thegroup consisting of calcined coke, petroleum coke, coal derived coke,calcined anthracite coal or mixtures thereof.
 10. The process of claim 1wherein the particulate fraction comprises up to about 65% of a fillercomprising at least about 75% coke having a diameter such that at leastabout 70% will pass through a 200 Tyler mesh screen.
 11. The process ofclaim 10 wherein the coke in the filler has a diameter such that atleast about 90% will pass through a 200 Tyler mesh screen.
 12. Theprocess of claim 10 wherein the filler comprises between about 0.5% andabout 25% of additives.
 13. The process of claim 12 wherein theadditives comprise iron oxide having an average particle diameter suchthat they are smaller than about 10 microns, petroleum coke having anaverage particle diameter such that they are smaller than about 10microns, and combinations thereof.
 14. The process of claim 1 whereinthe particulate fraction comprises at least about 50 weight percentcoke, coal or mixtures thereof having a diameter such that a majorfraction passes through a 25 mm mesh screen but not a 0.25 mm meshscreen.